1. Field of the Invention
The present invention relates to a semiconductor nonvolatile memory element that can change a threshold voltage through an externally applied electrical signal and a manufacturing method thereof.
2. Description of the Related Art
An electronic circuit used in electronic equipment is driven by a power supply such as a battery. When a voltage of the power supply fluctuates, malfunction of the electronic circuit and various abnormal phenomena may be caused. Thus, it is a typical approach to place a power management IC between the electronic circuit and the power supply, which is configured to regulate the voltage so that a constant voltage is output or monitoring fluctuations of the power supply, to thereby promote stable operation. In particular, in a semiconductor integrated circuit device such as a microcomputer or a CPU that is operated at increasingly lower voltages in recent years, the power management IC has been strongly required to output an accurate constant voltage and to accurately monitor the voltage value.
Exemplary power management ICs configured such that a constant voltage is output from a power supply to an electronic circuit include a step-down series regulator as illustrated in FIG. 3.
In this semiconductor integrated circuit device, a power supply voltage that is applied between a ground terminal 105 and a power supply terminal 106 is divided by a PMOS output element 104 and a voltage dividing circuit 103 including resistance elements 102. The voltage divided by the resistance elements 102 is input to a minus input terminal of an error amplifier 101, and is compared to a certain reference voltage value generated by a reference voltage circuit 100. Depending on a result of the comparison, the error amplifier 101 controls an input voltage of the PMOS output element 104 to change a source-drain resistance of the PMOS output element 104. As a result, an output terminal 107 has the function of outputting a constant output voltage that does not depend on the power supply voltage, but depends on the reference voltage value of the reference voltage circuit 100 and a resistance divided voltage ratio of the voltage dividing circuit 103. The output voltage is calculated by the following Expression (1):(Output voltage)=(reference voltage value)×(resistance divided voltage ratio of voltage dividing circuit)  (1)
In regulating the output voltage, by changing a resistance value of the resistance element 102 in a method described below, the divided voltage ratio of the voltage dividing circuit 103 is changed to set the output voltage value at a desired value based on Expression (1). Accordingly, the voltage dividing circuit of the semiconductor integrated circuit is required to be processed/corrected for each target output voltage.
Further, a voltage detector as illustrated in FIG. 4 that has the function of outputting a signal when the power supply voltage becomes a constant voltage is also one kind of the power management IC.
In this semiconductor integrated circuit device, the power supply voltage that is input from the power supply terminal 108 is converted to a voltage divided by the voltage dividing circuit 103 that includes the resistance elements 102, and the converted voltage is compared to the reference voltage value of the reference voltage circuit 100 by a comparator 108. A voltage signal corresponding to a result of the comparison is output from the output terminal 107. With this mechanism, a voltage detector is realized that has the function of monitoring the power supply voltage and outputting, when the voltage becomes equal to or higher than, or, equal to or lower than a certain voltage, a signal for the purpose of performing appropriate processing.
Also in the example illustrated in FIG. 4, by changing the resistance value of the resistance element 102, the divided voltage ratio of the voltage dividing circuit 103 is changed to set a desired voltage detection value based on Expression (1). Accordingly, the voltage dividing circuit of the semiconductor integrated circuit device is required to be processed/corrected for each target output voltage.
As a resistance element that is used for a voltage dividing circuit of a semiconductor integrated circuit device, a diffused resistor that is a monocrystalline silicon semiconductor substrate implanted with impurities having a conductivity type opposite to that of the semiconductor substrate, a resistor formed of polycrystalline silicon implanted with impurities, or the like is used. In designing the voltage dividing circuit, when a plurality of resistors are used, the resistors are set so as to have the same length, the same width, and the same resistivity. Then, the respective resistance elements are equally subjected to variations in shape in an etching process in which the shape is determined and to variations in impurity implantation. Accordingly, even if the absolute values of the resistance elements vary, resistance ratios between the resistance elements can be maintained at a constant value.
FIG. 5 is an illustration of a case in which the resistance elements having a certain resistance value based on the same shape and the same resistivity are used in a voltage dividing circuit. Various resistance values are realized through series connection and parallel connection of unit resistance elements 200 such as resistor groups 201 to 204 in FIG. 5. As described above, the unit resistance elements 200 are resistance elements having the same shape and the same resistivity, and thus, the high resistance ratios between the resistor groups each including the unit resistance element (s) having the high resistance ratio can be maintained with high accuracy.
Further, fuses 301 to 304 of, for example, polycrystalline silicon, are formed in parallel with the resistor groups 201 to 204, respectively, so as to be cut by laser radiation from the outside. Depending on whether or not the fuses are cut by the laser radiation, a resistance value between a terminal 109 and a terminal 110 can be changed as necessary. Then, a voltage corresponding to a divided voltage ratio to a fixed resistor formed between the terminal 110 and a terminal 111 is output from the terminal 110.
In the voltage dividing circuit as described above that has a highly accurate resistance ratio, by cutting the polycrystalline silicon fuse(s) with a laser, a desired divided voltage ratio can be obtained with high accuracy, and products having various target output voltages can be manufactured using the same semiconductor integrated circuit device.
A typical method of regulating an output voltage is as illustrated in FIG. 2.
First, an output voltage of a product completed in a semiconductor processing factory is measured as it is ((1) in FIG. 2). Then, based on a computational expression or a database prepared in advance depending on the output voltage, the polycrystalline silicon fuses formed in the voltage dividing circuit are processed with a laser to trim the output voltage ((2) in FIG. 2). Finally, the output voltage of the processed product is measured again to see whether or not the product is within specification as desired ((3) in FIG. 2). If the product is out of specification, the product is not shipped. Other than this, there is an online trimming method in which the resistors are gradually processed while the output voltage is monitored, and the processing is stopped when the output voltage reaches a desired value. The method illustrated in FIG. 2 is called an offline trimming method in contrast with the online trimming method.
Next, a reference voltage circuit that is used similarly in the circuits illustrated in FIG. 3 and FIG. 4 is described with reference to FIG. 6A and FIG. 6B.
A most basic related-art reference voltage circuit includes a depression type NMOS transistor 402 and an enhancement type NMOS transistor 401. As illustrated in FIG. 6A, each of the transistors is formed on a P-type well region 5 in a semiconductor substrate 1, and includes a gate electrode 6, a gate insulating film 9, and an N-type source/drain region 12. The transistors are different from each other in that, as an impurity region for determining a threshold voltage that is formed under the gate insulating film 9, an N-channel impurity region 10 is formed with regard to the depression type NMOS transistor 402 while a P-channel impurity region 11 is formed with regard to the enhancement type NMOS transistor 401. Further, each of the transistors includes a drain terminal 2 and a source terminal 3 for controlling operation thereof, and a body terminal 4 for fixing a potential of the P-type well region.
By connecting in series the depression type NMOS transistor 402 and the enhancement type NMOS transistor 401 between a power supply terminal 403 and a ground terminal 404 as illustrated in FIG. 6B, outputting a constant current from the depression type NMOS transistor 402 as a current source, and inputting the current to the drain terminal 2 of the enhancement type NMOS transistor 401 as a load element, a voltage generated at the drain terminal of the enhancement type NMOS transistor 401 that is a constant voltage is output to a reference voltage output terminal 405 (see, for example, Japanese Patent Application Laid-open No. 2008-198775).
The constant voltage that is output from the reference voltage circuit in this case is as expressed by the following Expression (2):(Reference voltage circuit constant voltage)=√(Ktd/Kte)×|Vtd|+Vte  (2),where Vtd and Ktd are a threshold voltage and a transconductance, respectively, of the depression type NMOS transistor, and Vte and Kte are a threshold voltage and a transconductance, respectively, of the enhancement type NMOS transistor.
In other words, variations in output voltage in Expression (1) arise from variations in parameters that determine the constant voltage that is output from the reference voltage circuit. The variations are absorbed through regulation of the resistance divided voltage ratio of the voltage dividing circuit.